Micromachining technology compatible with semiconductor processes is used to produce a number of devices such as piezoelectric motors containing cantilever beams, hinges, accelerometers, reflector antennae, microsensors, microactuators, and micromirrors, for example. One of the most popular microactuators is an electrostatic comb driver, due to its simplicity in fabrication and low power consumption. Surface micromachining fabrication processes for the electrostatic comb driver, as well as other beams and lever arms, have problems with stiction of such beams and lever arms to an underlying layer over which the beam or arm extends. The lever arm becomes deformed from its intended position, so that it does not extend out as desired. In the case of a membrane or diaphragm, the membrane or diaphragm becomes deformed from its intended position and may become stuck to an adjacent surface. Stiction is the number one yield-limiting problem in the production of the kinds of devices described above.
FIGS. 1A through 1C are simple schematics showing a cross-sectional side view of a starting structure for surface machining of a lever arm, the desired machined lever arm, and a lever arm which has been rendered non-functional due to stiction, respectively.
The FIG. 1A structure shows a substrate layer 102 (typically single crystal silicon), a portion of which is covered with a sacrificial layer 104 (typically silicon oxide), and a lever arm layer 106 (typically polysilicon) which is in contact with and adhered to substrate layer 102 at one end of lever arm layer 106. FIG. 1B shows the FIG. 1A structure after the removal of sacrificial layer 104 to produce the desired free-moving lever arm 107. The height “h” of gap 108 between lever arm 107 and substrate 102, the length “l”, and the cross-sectional thickness “t” of the lever arm 107 depend on the particular device in which the structure is employed. In many instances the relative nominal values of “h”, “l”, and “t” are such that capillary action during the fabrication process; or contaminants formed as byproducts of the fabrication process; or van der Waals forces; or electrostatic charges on the upper surface 110 of substrate layer 102 and/or on the undersurface 112 of lever arm layer 106, may cause lever arm 106 to become stuck to the upper surface 110 of substrate layer 102. This problem is referred to as “stiction”, and is illustrated in FIG. 1C. Stiction may occur during formation of the lever arm 107, or may occur subsequent to fabrication of the device and during packaging, shipment, or use (in-use stiction) of the device. A single crystal silicon or polysilicon surface of the kind which is frequently used to fabricate a lever arm, beam, membrane, or diaphragm is hydrophilic in nature, attracting moisture, which may cause stiction.
Stiction, which is the primary cause of low yield in the fabrication of MEMS devices, is believed to result from a number of sources, some of the most significant being capillary forces, surface contaminants, van der Waals forces, and electrostatic attraction. Factors which may contribute to stiction include: warpage due to residual stresses induced from materials; liquid-to-solid surface tension which induces collapse; drying conditions during processing; adverse and harsh forces from wet baths; aggressive designs (i.e., long and thin beams); surface-to surface attractions; inadequate cleaning techniques; aggressive cleaning techniques; and environments subsequent to fabrication, including packaging, handling, transportation, and device operation.
Various processes have been developed in an attempt to prevent stiction from occurring during fabrication of micromachined arms and beams. To reduce the possibility of stiction subsequent to release of a beam, lever arm, membrane, or diaphragm (so that it extends over open space), a surface treatment may be applied and/or a coating may be applied over freestanding and adjacent surfaces. For example, in U.S. Pat. No. 6,096,149, to Hetrick et al, issued Aug. 1, 2000, the inventors disclose a method for fabricating an adhesion-resistant microelectromechanical device. Amorphous hydrogenated carbon is used as a coating or structural material to prevent adhesive failures during the formation and operation of a microelectromechanical device. (Abstract) The amorphous hydrogenated carbon (AHC) coating is applied on the micromachined device after removal of the sacrificial layer and release of the structure. The sacrificial layer is removed in a wet etching solution such as hydrofluoric acid or buffered HF acid. (Col. 7, lines 26-32.) The method is said to reduce adhesive forces between microstructure surfaces by altering their surface properties. The AHC is said to create a hydrophobic surface, which results in lower capillary forces and an associated reduction in stiction. (Col. 2, lines 66-67, continuing at Col. 3, lines 1-4.)
U.S. Pat. No. 5,403,665, issued Apr. 4, 1995, to Alley et al., discloses a method of applying a self-assembled alkyltrichlorosilane monolayer lubricant to micromachines. Octadecyltrichlorosilane (OTS; C18H37SiCl3) is provided as an example of an alkyltrichlorosilane. In a dilute, non-polar, non-aqueous solution, OTS will deposit on silicon, polysilicon, and silicon nitride surfaces that have been previously treated to form a hydrophilic chemical oxide. Treatment of the silicon, polysilicon, or silicon nitride surfaces may be accomplished with an approximately 5 to 15 minute exposure to a hydrophilic chemical oxide promoter such as Piranha (H2O2:H2SO4), RCA SC-1, or room temperature H2O2. This treatment changes silicon and polysilicon surfaces from hydrophobic to hydrophilic. Thus, the surface will have a thin layer of adsorbed water. The OTS reacts with the thin adsorbed water layer that is present on the treated surface to form a single layer of molecules that are chemically bonded to the surface. (Col. 3, lines 23-40; Col. 4, lines 19-30)